In recent years, a wireless communication system utilizing a RFID (Radio Frequency IDentification) technique has been developed as a contactless-type tag. A RFID tag is required to be produced at low cost. Therefore, it is the common practice to provide no power source on the tag. An antenna installed in the tag receives a carrier wave sent from an antenna in an external reader/writer and induces an electromotive force through electromagnetic induction, and a rectifier circuit in the tag generates a direct current from an alternating current induced in the rectifier circuit. In this manner, a wireless power source can be produced.
It is critical for the rectifier circuit to be installed in the RFID tag to produce a maximum direct-current output voltage by a minimum alternating-current input voltage signal amplitude. For this purpose, the rectifying element constituting the rectifier circuit is required to have high power conversion efficiency, i.e., low electric power loss.
The underlying cause of the electric power loss in a rectifying element is the loss of an electric power which is caused by the electric resistance of the rectifying element. Therefore, in order to reduce the electric power loss, it is critical to reduce the forward resistance of the rectifying element. Therefore, it has been widely studied to use a member capable of achieving this reduction as a semiconductor layer in a rectifying element.
On the other hand, a carbon nano material having a sharp-pointed tip shape and a high aspect ratio, typically a carbon nanotube (also referred to as “CNT”, hereinafter), has high electrical conductivity. Many types of carbon nano materials have been developed as semiconductor materials and materials for fuel cells, and have also been used in rectifying elements. A CNT is expected to be used as a rectifying element that can be operated at a high frequency, because a CNT has good electrical properties, rapid responsibility and the like. A CNT is also expected to be reduced in size and to be installed at high density.
However, for reducing the forward resistance of a rectifying element utilizing CNTs and improving the stability of the element, it is critical to disperse multiple CNTs uniformly in a semiconductor layer in the rectifying element. Then, a technique has been proposed, which can produce a stable rectifying element using a CNT structure having a structure such that multiple CNTs form a network structure through crosslinked sites (see, for example, Patent Document 1).